1. Field of the Invention
This invention relates to the fabrication of magnetic read/write heads that employ TAMR (thermally assisted magnetic recording) to enable writing on magnetic media having high coercivity and high magnetic anisotropy. More particularly, it relates to the use of a planar plasmon generator (PPG) to transfer the required thermal energy from the read/write head to the media by means of a propagating surface plasmon mode.
2. Description of the Related Art
Magnetic recording at area data densities of between 1 and 10 Tera-bits per in2 involves the development of new magnetic recording media, new magnetic recording heads and, most importantly, a new magnetic recording scheme that can delay the onset of the so-called “superparamagnetic” effect. This latter effect is the thermal instability of the extremely small regions of magnetic material on which information must be recorded, in order to achieve the required data densities. A way of circumventing this thermal instability is to use magnetic recording media with high magnetic anisotropy and high coercivity that can still be written upon by the increasingly small write heads required for producing the high data density. This way of addressing the problem produces two conflicting requirements:    1. The need for a stronger writing field that is necessitated by the highly anisotropic and coercive magnetic media.    2. The need for a smaller write head of sufficient definition to produce the high areal write densities, which write heads, disadvantageously, produce a smaller field gradient and broader field profile.
Satisfying these requirements simultaneously may be a limiting factor in the further development of the present magnetic recording scheme used in state of the art hard-disk-drives (HDD). If that is the case, further increases in recording area density may not be achievable within those schemes. One way of addressing these conflicting requirements is by the use of assisted recording methodologies, notably thermally assisted magnetic recording, or TAMR.
Prior art forms of assisted recording methodologies being applied to the elimination of the above problem share a common feature: transferring energy into the magnetic recording system through the use of physical methods that are not directly related to the magnetic field produced by the write head. If an assisted recording scheme can produce a medium-property profile to enable low-field writing localized at the write field area, then even a weak write field can produce high data density recording because of the multiplicative effect of the spatial gradients of both the medium property profile and the write field. These prior art assisted recording schemes either involve deep sub-micron localized heating by an optical beam or ultra-high frequency AC magnetic field generation.
The heating effect of TAMR works by raising the temperature of a small region of the magnetic medium to essentially its Curie temperature (TC), at which temperature both its coercivity and anisotropy are significantly reduced and magnetic writing becomes easier to produce within that region.
In the following, we will address our attention to a particular implementation of TAMR, namely the transfer of electromagnetic energy to a small, sub-micron sized region of a magnetic medium through interaction of the magnetic medium with the near field of an edge plasmon excited by an optical frequency laser. The transferred electromagnetic energy then causes the temperature of the medium to increase locally.
The edge plasmon is excited in a particularly shaped plasmon generator (PG) that is incorporated within the read/write head structure. The source of optical excitement can be a laser diode, also contained within the read/write head structure, or a laser source that is external to the read/write head structure, either of which directs its beam of optical radiation at the PG through a means of intermediate transfer such as an optical waveguide (WG). As a result of the WG, the optical mode of the incident radiation couples to a propagating edge plasmon mode in the PG, whereby the optical energy is converted into plasmon energy that travels along the PG. This plasmon energy is then focused by the PG onto the medium, at which point the heating occurs. When the heated spot on the medium is correctly aligned with the magnetic field produced by the write head pole, TAMR is achieved.
The following prior arts describe such TAMR implementations, some of which are in the form of an edge plasmon generator (EPG) structure having a triangular cross-section in a plane perpendicular to the direction of plasmon propagation (hereinafter denoted the y-z plane).
K. Tanaka et al. (US Publ. Pat. App. 2008/0192376) discloses a thermally assisted magnetic head.
K. Shimazawa et al. (US Publ. Pat. Appl. 2008/0198496) discloses a near-field light generator plate incorporated within a TAMR head in a HDD assembly.
Y. Zhou et al. (US Publ. Pat. Appl. 2010/0315735) discloses a plasmon antenna with a magnetic core for thermally assisted magnetic recording.
Buechal et al U.S. Pat. No. 7,596,072 teaches that the optical spot dimensions are determined by the dimensions of a small metal structure inside the head. Theoretically, the optical spot can be as small as 20 nm.
Chou et al. (U.S. Patent Application 2011/0090587) describes a plasmon generator that has a very small spot beam.
Shimazawa et al. (U.S. Pat. No. 7,940,486) and Tanaka et al. (U.S. Patent Applications 2008/0192376) disclose plasmon antennas having a triangle shape and made of a conductive material.
Kamura et al. (U.S. Patent Application 2011/0205661) also shows a plasmon generator having a triangular shape.
When a properly shaped prior art EPG is placed in the vicinity of an optical waveguide, it will support a highly confined edge plasmon (EP) mode. By means of evanescent coupling between the optical mode in the WG and the edge plasmon mode in the EPG, the optical energy in the WG can be efficiently transferred to the EPG mode, which then propagates (along the x-direction hereinafter) towards the ABS where it delivers the optical energy, and where it locally heats the recording medium placed beneath the EPG. The EPG is made of noble metals, such as Ag and Au, which are known to be excellent at generating optically driven surface plasmon modes. The local confinement of the edge plasmon mode within the EPG is determined by the angle and radius of the triangular EPG corner, the noble metal forming the EPG and the dielectric material surrounding the tip of the EPG.
Referring to FIG. 1, there is shown the result of optically modeling the cross-track dependency of the optical spot size on the tip radius in the case of 90° gold prior art design EPG. Note that the ordinate measures optical spot size in the cross-track dimension, while the abscissa measures the tip radius of the EPG. For a 25 nm tip radius the optical spot size in the medium about 100 nm. Even with a 5 nm tip radius it is difficult to obtain an optical spot size that is less than 50 nm in the medium, which is a requirement for the first generation of TAMR products. Reducing the angle of the EPG can reduce the spot size to a small extent, but both the coupling efficiency between the WG and EPG and the propagating efficiency of the EP mode will be greatly reduced due to a higher mode index and a higher damping loss of the narrower tip angle. Neither of these results are desirable and they increase the concern over EPG reliability. In addition, the level of process control required to produce such a sharp tip angle is itself challenging.
In the thermally dominant TAMR scheme, the properties of the written bit strongly depends on both the thermal spot size and shape in the recording layer and the alignment between the magnetic gradient and the thermal gradient. Therefore, it is very desirable to be able to consistently reduce the optical spot size in the recording medium by means of a PG structure with well-defined and scalable features that could support a few generations of TAMR product development. Presumably such an evolution of the TAMR product will require that features be scaled down in size, while not adversely affecting the properties of the written bit. The PG structure should also not only possess this scalability, but it should be relatively easy to fabricate and the fabrication process should be easily and well controlled.
None of these issues are addressed by the prior arts cited above. However they will be dealt with by the present invention, as will now be described in greater detail.